Filter medium for filtering molten metals

Abstract

Filter medium in the form of a stable body made of ceramic material and employed for filtering molten metals. The filter medium is made up of at least two effective filter layers such that a relatively thick layer of large pore diameter lies on a relatively thin layer of small pore diameter. This arrangement aims at attaining the maximum possible wetting of the interior of the filter medium.

What is claimed is: 
     
       1. A filter medium in the form of a stable body of ceramic material for filtering molten metal comprising at least a first filtering layer A and a second filtering layer B arranged one after the other in the direction of filtration, said at least first filtering layer A having a thickness d A  of between 9-91 mm, porosity f A  of between 0.25-0.8 and pore diameter λ A  of between 0.5-2 mm wherein the parameter H SA  of layer A which is proportional to the filtering resistance of layer A is equal to ##EQU4## and has values of between 28-14,560 cm -1 , said at least second filtering layer B having a thickness d B  of between 1-33 mm, porosity f B  of between 0.25-0.5 and pore diameter λ B  of between 0.2-1 mm wherein the parameter H SB  of layer B which is proportional to the filtering resistance of layer B is equal to ##EQU5## and has values of between 20-33,000 cm -1 , and wherein the ratio of thickness d A  to thickness d B  lies between 9:1 and 3:1, and the ratio of the numerical values of parameters H SA  and H SB  lies between 1:1.2 and 1:30. 
     
     
       2. A filter medium according to claim 1 wherein said first filtering layer A is positioned upstream of said second filtering layer B. 
     
     
       3. A filter medium according to claim 1 wherein said first filtering layer A and said second filtering layer B are made up of refractory material in the form of spherical grains bonded together. 
     
     
       4. A filter medium according to claim 3 wherein said spherical grains are hollow. 
     
     
       5. A filter medium according to claim 4 wherein said refractory material comprises hollow, spherical grains of corundum. 
     
     
       6. A filter medium according to claim 5 wherein said hollow corundum grains are employed with close control on grain size distribution such that the minimum grain diameter in a range of particle sizes is greater than 65% of the size of the maximum particle size. 
     
     
       7. A filter medium according to claim 3 wherein said grains are strongly bonded together with a glassy type binder, the amount of binder accounting for 20-40 wt. % of the final filter medium. 
     
     
       8. A filter medium according to claim 3 wherein said grains are bonded strongly together by a crystalline substance. 
     
     
       9. A filter medium according to claim 1 wherein the filter medium is built up of three layers wherein at least one of the layers contains a fluoride bearing compound.